The development of silane terminated polyether and polyurethane polymers to replace standard isocyanate capped polyurethane prepolymers in one component moisture curing compositions have, in the last 15 years, shown that these polymers are of lower toxicity, are easy to process and are a good alternative polymer to replace Polyurethane (PU) prepolymers. The new higher molecular weight double metal cyanide complex catalyst (DMC)-containing low monol polyether polyols, produced by most manufacturers, have also made a big improvement in the physical properties of these silane cross linked polymers. There has now been substantial replacement of solvent and some free isocyanate monomer containing sealants and adhesives in many market segments using traditional toluene diisocyanate (TDI) or methylene diphenyl diisocyanate (MDI) based PU adhesives. The advantages of the good UV resistance provided by these silane modified polyether polymers has been very significant and is due to the absence of any aromatic molecular groups. These difunctional methoxy silane capped polyether polymers have easy processing properties because the reinforcing precipitated calcium carbonates and carbon black fillers can be processed undried with the polymer and then dried with a moisture scavenger such as vinyl trimethoxy silane. This is a great advantage over polyurethane polymers that are an isocyanate group (NCO) terminated prepolymer, where the powders need to be very dry before mixing with the prepolymers. There is however a minor disadvantage due to possible methanol emission on curing and high catalyst and amino silane content needed for fast curing adhesives.
An alternate polymer design where an isocyanate terminated prepolymer is capped with a secondary amino propyl trimethoxy silane (of various types) has shown the benefits of better crosslinking of the trimethoxy silane and the low monol DMC process with the use of high molecular weight polyether. This type of polymer design allows the use of high di-isodecyl phthalate (DIDP) plasticizer content as used with standard PU prepolymers and can produce sealants and adhesives with good elastic recovery and low tin catalyst content. Therefore, this type of polymer design is a good alternate to PU prepolymers. However, a major disadvantage of this polymer design is its high polymer viscosity in the range of 50,000 Mpas to 100,000 Mpas, which necessitates the use of plasticizer contents of at least 20 percent in the polymer to make the polymer process functional in a sealant factory. Due to this high viscosity, drums of these polymers are impossible to empty satisfactorily. Additionally, high plasticizer content in the formulation can result in soft sealants and a tacky surface on cure. Furthermore, moisture sensitivity of the trimethoxy silanes is an issue in manufacturing and these polymers must be processed with completely dry calcium carbonates, or alternately, the calcium carbonates must be dried with the plasticizer before mixing them into the polymer.
The composition of silane terminated polyurethanes is well known, and nearly all diisocyanate molecules are capable to being reacted partially with a hydroxyl functional polymer and then the free NCO groups capped with a secondary amino silane. The preferred diisocyanates are the aliphatic diisocyantes, and Hexamethylene diisocyanate (HDI) and Isophorone Diisocyanate (IPDI), however, there are many combinations known to practitioners of the art.
The high viscosities of PU prepolymers and also the secondary amino silane capped polyurethane polymers are due to the hydrogen bonding developed between the urethane and urea groups where the double bonded oxygen atom and the hydrogen on the nitrogen atoms in adjacent polymer chains form a cross-chain hydrogen bond. The increased viscosity can be reduced in a urea by adding a bulky group to the nitrogen of the secondary amino silane and the use of phenyl, cyclohexyl, aspartame ester and diethyl maleate or octyl maleate and other groups is documented and is well-known. Due to the fact that the polymer chains and their associated hydrogen bonding exist in three dimensions, viscosity reducing methods can vary in their efficiencies. Some methods have employed adding propylene carbonate and long chain alcohols as a plasticizer to reduce viscosity of these silane terminated polyurethane prepolymers where secondary amino silanes are used. However, there is a need for improvement.
A replacement is needed for standard TDI and MDI based prepolymers that contain solvents like xylene or toluene in the prepolymer, along with some free NCO containing monomer that is very difficult to remove. This replacement should provide formulators and their factories a safer low toxicity and lower viscosity polymer. Additionally, a method is needed where formulators are able to manufacture sealants in a very similar process to what is currently used with current prepolymers and formulations. The formulators need stable thinner polymers, moisture scavengers and cure inhibitors enabling the formulation of products that can be manufactured in equipment that is normally used for high volume PU sealant production. The solution should be able to replace current proven PU sealant formulations with less development and advantages in physical properties such as UV exposure resistance and better adhesion with no primers.
Although there are many known advantages of methyl dimethoxy silane end capping of polyurethanes, such as having low moisture sensitivity and an easier manufacturing process, the viscosity of these polymers is still a major problem. This issue needs to be addressed in order to make such polymers commercially viable and the present method provides some solutions to that problem.
What is needed is a method of producing a low viscosity dimethoxy amino silane polyurethane with triethoxy silyl groups, which can reduce the problems associated with the current use of methyl dimethoxy silane terminated end capped polyurethanes, in order to facilitate the manufacture of sealants, adhesives and paint compositions.